This application relates generally to fiber-optic communications and more specifically to techniques and devices for routing different spectral bands of an optical beam to different output ports (or conversely, routing different spectral bands at the output ports to the input port).
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future.
In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronicsxe2x80x94typically an electronic SONET/SDH system. However SONET/SDH systems are designed to process only a single optical channel. Multi-wavelength systems would require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology.
The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called xe2x80x9cwavelength routing networksxe2x80x9d or xe2x80x9coptical transport networksxe2x80x9d (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. New types of photonic network elements operating at the wavelength level are required to perform the cross-connect, ADM and other network switching functions. Two of the primary functions are optical add-drop multiplexers (OADM) and wavelength-selective cross-connects (WSXC).
In order to perform wavelength routing functions optically today, the light stream must first be de-multiplexed or filtered into its many individual wavelengths, each on an individual optical fiber. Then each individual wavelength must be directed toward its target fiber using a large array of optical switches commonly called as optical cross-connect (OXC). Finally, all of the wavelengths must be re-multiplexed before continuing on through the destination fiber. This compound process is complex, very expensive, decreases system reliability and complicates system management. The OXC in particular is a technical challenge. A typical 40-80 channel DWDM system will require thousands of switches to fully cross-connect all the wavelengths. Opto-mechanical switches, which offer acceptable optical specifications are too big, expensive and unreliable for widespread deployment. New integrated solid-state technologies based on new materials are being researched, but are still far from commercial application.
Consequently, the industry is aggressively searching for an all-optical-wavelength routing solution which enables cost-effective and reliable implementation of high-wavelength-count systems.
The present invention provides a wavelength router that allows flexible and effective routing of spectral bands between an input port and a set of output ports (reversibly, also between the output ports and the input port).
An embodiment of the invention includes a free-space optical train disposed between the input port and the output ports. The free-space optical train may include air-spaced elements or may be of generally monolithic construction. The optical train includes a transmissive dispersive element, such as a transmissive diffraction grating, disposed so that light is intercepted from the input port and encounters the transmissive dispersive element at least four times before reaching any of the output ports. Certain embodiments also include a routing mechanism having at least one dynamically configurable routing element, which cooperates with elements in the optical train to provide optical paths that couple desired subsets of the spectral bands to desired output ports. The routing elements are disposed to intercept the different spectral bands after they have been spatially separated by the transmissive dispersive element.
In certain embodiments, the transmissive dispersive element is encountered by light in pairs, with a reflective surface being disposed to reflect light immediately back towards the transmissive dispersive element after it has propagated through the transmissive dispersive element. The reflective surface and the transmissive dispersive element may be oriented with respect to an optical axis of the wavelength router so that each encounter with the transmissive dispersive element is near the Littrow condition. The reflective surface may be flat, although in other embodiments it comprises optical power. In a specific embodiment, the transmissive dispersive element and the reflective surface are comprised by an integrated element.
The invention includes dynamic switching embodiments and static embodiments. In dynamic embodiments, the routing mechanism includes one or more routing elements whose state may be dynamically changed in the field to effect switching. In static embodiments, the routing elements are configured at the time of manufacture or under circumstances where the configuration is intended to remain unchanged during prolonged periods of normal operation.
In the most general case, any subset of the spectral bands, including the subset that consists of no spectral bands and including the subset that consists of the whole set of spectral bands, can be directed to any of the output ports. However, there is no requirement that the invention be able to provide every possible routing. Further, in general, there is no constraint on whether the number of spectral bands is greater or less than the number of output ports.
In some embodiments, the routing mechanism includes one or more retroreflectors, each disposed to intercept a respective one of the spectral bands after twice encountering the transmissive dispersive element, and direct the light in the opposite direction with a controllable transverse offset. In other embodiments, the routing mechanism includes one or more tiltable micromirrors, each of which can redirect one of the spectral bands with a controllable angular offset. There are a number of ways to implement the retroreflectors.
In some embodiments, the beam is collimated before encountering the transmissive dispersive element, so as to result in each spectral band leaving after twice encountering the dispersive element as a collimated beam traveling at an angle that varies with its wavelength. The dispersed beams are then refocused onto respective routing elements and directed back so as to encounter the same elements in the optical train and to encounter the transmissive dispersive element twice before exiting the output ports as determined by the disposition of the respective routing elements. Some embodiments of the invention use cylindrical lenses while others use spherical lenses.